Link relations in HTTP Link header conflict with RFC 5988.

IndieAuth [registers](https://indieauth.spec.indieweb.org/#iana-considerations) 3 link relations: `authorization_endpoint`, `token_endpoint`, and `redirect_uri`. Additionally it requires both [clients](https://indieauth.spec.indieweb.org/#discovery-by-clients) and [servers](https://indieauth.spec.indieweb.org/#redirect-url) to check HTTP `Link` headers for URLs with these relations.

According to [RFC 5988](https://tools.ietf.org/html/rfc5988) (as cited by IndieAuth) an `_` (underscore) cannot be used by a link relation within [the header field](https://tools.ietf.org/html/rfc5988#section-5). A link’s relation type must match:

> ```
> relation-type = reg-rel-type | ext-rel-type
> reg-rel-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
> ext-rel-type = URI
> ```

[RFC 8288](https://tools.ietf.org/html/rfc8288) (which replaces RFC 5988, cf. #15) does not solve this for us. There a link’s [relation type](https://tools.ietf.org/html/rfc8288#section-3) is defined using a slightly different ABNF but otherwise unchanged:

> ```
> relation-type = reg-rel-type / ext-rel-type
> reg-rel-type = LOALPHA *( LOALPHA / DIGIT / "." / "-" )
> ext-rel-type = URI ; Section 3 of [RFC3986]
> ```

Start by reading the IndieAuth IANA considerations, client discovery, and redirect URL sections alongside RFC 5988 and RFC 8288. No implementation files or tests are named, so first locate where these link relations are defined and parsed. Done means the specification consistently defines relation names that conform to the applicable RFC grammar.

Written by the indexing model from the issue text.